Tapasin is a 48kD glycoprotein which is a member of the immunoglobulin (Ig) superfamily and it is required for efficient peptide binding to Transporter associated with Antigen Processing (TAP). TAP binds peptides in its cytolic part and subsequently translocates the peptides into the lumen of the endoplasmic reticulum (ER) where assembly of MHC class I and peptide takes place. Assembly of MHC class I-beta2-microglobulin (beta2-m) dimers in the ER involves 2 chaperones, calnexin which interacts with free class I heavy (H) chains and calreticulin which binds human class I-beta2 dimers prior to peptide loading. Calreticulin remains associated with at least a subset of class I molecules when they in turn bind to TAP (1). Polymorphic differences in MHC class I H chains can result in quantitative as well as qualitative differences in how they interact with peptide, beta2-m, calnexin, calreticulin, Erp57, TAP and Tapasin, a subunit of the TAP complex which binds to both TAP1 and MHC class I (2,3). Data obtained with Tapasin deletion mutants revealed that binding to TAP is mediated by the C-terminal region and that the N-terminal region is required to stabilize the MHC class I loading complex (4). The Tapasin gene is centromeric of HLA-DP locus between the HSET and HKE1.5 genes and within 500kbp of the transporters associated with antigen processing, TAP1 and TAP2 genes. The localization of these genes within such a short distance of each other on the chromosome implies some regulatory or functional significance (5).